The present invention relates to radiator elements of the type used in radar systems such as active array and phased radar applications.
The principle radiating elements heretofore used for broadband active arrays have been the dielectric bilateral and all metalized flared notch radiators. These radiators are described in, e.g., "Broadband Antenna Study," L. R. Lewis and J. Pozgay, Final Report AFCRL-TR-75-0178, Air Force Cambridge Research Laboratories, March 1975; "Analysis of the Tapered Slot Antenna," R. Janaswamy and D. Schaubert, IEEE Trans. Antennas and Propagation, Vol. AP-35, No. 9, September 1987, pages 1058-1059; "The Vivaldi Aerial," P. J. Gibson, Proceedings of the Ninth European Microwave Conference, 1979, at pages 101-105. Because of the coplanar nature of their slotline-type configuration, both of these radiators require balun transitions from stripline-type transmission line to the slotline flare notch in order to launch the RF signal from the stripline or microstrip mode to the slotline mode. The need for baluns tends to limit very wide band performance. The presence of the balun also tends to make the packaging more complicated and more costly.
Prior approaches to integrating a circulator or any other component to such radiator elements would be to first connect the component to the stripline portion of the balun which then transitions to the flared notch. This connection is either a direct connection or with the addition of some type of coaxial connector interface, with the attendant disadvantages that the structure is more difficult to assemble and with the possible degradation of the match.
The antipodal flared notch radiator, described in "Improved design of the Vivaldi antenna," by E. Gazit, IEE Proc., Vol. 135, Pt.H, No. 2, April 1988, at pages 89-92, extends the concept of the Van Heuven microstrip to waveguide transition to antenna elements. The Van Heuven transition is described, e.g., in "A New Model for Broadband Waveguide-to-Microstrip Transition Design," G. E. Ponchak and Alan N. Downey, Microwave Journal, May, 1988, pages 333 et seq. FIG. 1 shows a top view of the antipodal flared notch radiator 20. FIGS. 2A-2F illustrate particular cross-sectional views of the radiator device of FIG. 1. The input microstrip line 22 is transformed into a broadside coupled strip 24 (odd mode needed only) by narrowing the groundplane. The broadside coupled strips 24 then are transformed into an antipodal slotline 26. Finally the antipodal slotline flares out as in the typical notch radiator. Note how the electric fields of the microstrip 22 are rotated and transformed into the electric fields of the slotline (FIGS. 2A-2F). Thus, FIG. 2A illustrates the field configuration of the input microstripline. FIG. 2B shows the transitioning of the microstripline to the broadside-coupled strips (FIG. 2C). FIG. 2D shows the field configuration at the antipodal slotline. FIG. 2E shows the transitioning from the antipodal slotline to the flared out structure near the radiator tip (FIG. 2F).
FIGS. 3A-3F show various slotline structures and the corresponding gaps G. FIG. 3A shows a conventional coplanar slotline structure. FIG. 3B shows a sandwiched coplanar slotline, i.e., where the conductor strip and groundplane are sandwiched between dielectric layers. FIG. 3C shows a coplanar thick metal slotline structure. FIG. 3D shows a bilateral coplanar slotline structure. FIG. 3E shows an antipodal slotline structure. FIG. 3F shows a sandwiched antipodal slotline structure.
The antipodal structure is more versatile than conventional coplanar or bilateral slotline structures because low impedances (characteristic impedance Z less than 60 ohms) can be realized more easily. Low impedances in conventional coplanar and bilateral slotlines require very narrow slot gap dimensions which are difficult to realize because of manufacturing tolerances. Low impedance in antipodal slotline are relatively easy to realize because it involves simply controlling the amount of overlap between the two conductors.
As shown in FIG. 1, there are no abrupt transitions or discontinuities to limit the bandwidth performance of the antipodal flared notch radiator element. All the transmission lines can be designed to be 50 ohms prior to entry into the flared region. Since there is no balun required, fabrication of this element is very simple and inexpensive because it involves only a single double-sided printed circuit board. One limitation of the conventional antipodal flared notch radiator is that the opening of the flared notch is a half-wavelength at the low end of the frequency band. As the low end of the frequency band is decreased, the physical size of the flared notch increases and may exceed the allowable physical space for some applications. Another limitation is that the conventional radiator has only a single port (microstripline 22) which must be used for both transmit and receive operations.
Because of its asymmetry, the antipodal flare notch radiator of FIG. 1 would be difficult to model analytically in an array, and will not image properly in waveguide simulators. Waveguide simulators, as is well known in the art, are test apparatus used to measure the active impedance of large or infinite arrays. Small clusters of radiating elements are placed in a waveguide, which acts as a mirror, simulating the performance of an infinite array. To work properly, the small cluster must be symmetric with respect to the walls of the waveguide.
Accordingly it is an object of this invention to provide a flared notch radiator element with separate transmit and receive ports.